This invention relates to a process for non-destructively evaluating the integrity and intrinsic performance characteristics of porous membranes such as ultrafiltration membranes. More particularly, this invention relates to a non-destructive testing process the quantitatively determining the effect of undesirable large pores in a membrane which, when present, drastically degrade the membrane's solute retention capabilities.
Ultrafiltration membranes have pores of a size typically between about 10 Angstroms and 0.05 microns. It is widely believed that the majority of pores present in ultrafiltration (UF) membranes (.about.80%) lie within a narrow range of the pore distribution. However, since the membrane permeability, and consequently, the transmembrane transport depend upon the fourth power of the pore radius, these smaller pores usually contribute less than 10% to the total membrane permeability. As a consequence, a very small number of large-sized pores actually control the membrane performance. For membranes whose retention of solute molecules is based upon a sieving mechanism, the evaluation of the "active" pore size distribution is of cardinal importance in predicting the selective transport through such membranes. Therefore, to accurately correlate solute retention, it is necessary to characterize these larger "transport controlling" pores.
The bubble point test and the air diffusion tests are two non-destructive integrity tests which have been previously employed in an attempt to correlate and predict solute retention for various classes of membranes. In the bubble point test, a thoroughly wetted membrane is placed into a housing and contacted with air. The upstream air pressure is then gradually increased, eventually resulting in the selective intrusion of air through the largest pores and the subsequent formation of air bubbles downstream of the membrane. Assuming cylindrical pores, the pore diameter corresponding to the pressure at which these bubbles are first observed (bubble point) can be approximated by the modified Young-Laplace capillary equation given by ##EQU1## where P is the transpore pressure drop, d.sub.p is the pore diameter intruded, .theta. is the contact angle, K represents a shape correction factor, and .gamma. is the air-liquid interfacial tension. Since capillary forces dictate that the largest pores are those first intruded with air, the bubble point test can be considered a measure of the largest pore present in the given membrane sample. It is the characterization and quantification of these largest pores which is essential for developing a test capable of correlating and predicting solute retention. However, two problems exist with the traditional bubble point test as it pertains to the evaluation of ultrafiltration membranes. First, due to the extremely large interfacial surface tension at the air/liquid interface, the pressures required to observe the bubble point for typical ultrafiltration (UF) membranes are in excess of 500 psi. Conventional UF membranes may compress when subjected to pressures in this range, leading to results that could be incorrectly interpreted. Second, as the membrane area to be tested increases, the actual membrane bubble point becomes more difficult to detect due to the large background of air diffusing through the wetted membrane. Although this problem can be minimized by utilizing a gas that has a low solubility in the wetting liquid used to intrude the pores, diffusion due to solubility effects can not be totally eliminated. Consequently, this test is essentially limited to small area microfilters.
In the air diffusion test, the membrane is again wetted and contacted with air. The air pressure is increased to some prescribed value below the membrane's bubble point and the total amount of air flow through the wetted membrane by diffusion and convection is recorded. Since the operating pressure is usually well below the bubble point of the membrane, integral membranes exhibit only diffusional flow. In fact, only gross defects present in the membrane sample, those which contribute measurable convective air flow, can be detected. Thus, on a theoretical basis, this test can not be expected to correlate well to the solute retention of integral membranes since this test has sensitivity only to gross defects.
A permoporometric technique for the characterization and pore size distribution determination of various classes of UF membranes is disclosed by:
"Membrane Morphology and Transport Properties", Desalination, 53 11 (1985),
"Computer Driven Porosimeter for Ultrafiltration Membranes", Characterisation of Porous Solids, K. K. Unger, et al., eds., 283 (1988),
"Permoporometric Study on Ultrafiltration Membranes", J. Membrane Sci., 41 69 (1989), and
"Correlation of Direct Porosimetric Data and Performance of Ultrafiltration Membranes", Proc. Biochem. Int'l., 111 (1990).
In this permoporometric technique, the air-liquid interface typically encountered in bubble point testing is now replaced with the interface between two immiscible liquids. The key advantage with utilizing a two phase liquid system is the extremely small interfacial tensions associated with many pairs of immiscible liquids, resulting in low transmembrane pressures necessary to selectively intrude nanometer sized pores. In addition, since the two phases are completely immiscible, there is no background diffusional flow to contend with, resulting in a technique which is linearly scalable and independent of membrane surface area. Thus, this technique is eminently suitable for the characterization of UF membranes.
In the disclosed permoporometric technique, a membrane sample is first wetted with one of two mutually immiscible liquid phases (wetting phase). The other immiscible liquid phase (intrusion phase) is then placed upstream of the membrane housing. The intrusion phase is then sequentially pumped through the membrane sample at prescribed flow rates and the resulting equilibrium upstream pressures recorded. As with the bubble point test, the first pores intruded at the lowest flow rates (lowest pressures) are the largest pores present in the membrane sample. Flowever, the use of two immiscible fluids with an extremely low interfacial surface tension has the advantage of requiting pressures less than 20 psi for the complete intrusion of these larger pores present in UF membranes. With knowledge of the upstream pressures corresponding to the various intrusion phase flow rates, the interfacial surface tension of the two immiscible fluids, and assuming the validity of a particular mathematical model, this test is able to calculate an effective pore size distribution of the membrane sample. Based on this entire calculated pore size distribution, the performance of conventional UF membranes was able to be correlated.
There are two major limitations associated with the disclosed technology as it pertains to rapidly and non-destructively correlating the particle retention capabilities of ultrafiltration membranes. First, the disclosed technology relies upon the generation of an entire effective pore size distribution for the tested membrane samples, an extremely tedious process which may take upwards of 2-3 hours or longer depending on the desired degree of accuracy. In addition, the two phase system disclosed by these references throughout the UF characterization experiments is an isobutanol: methanol:water (15:7:25 v/v/v) system. This solvent system may be difficult to remove from some conventional UF membranes and is toxic to most biological fluids. For these reasons, any test involving the use of this particular two phase system may render the test destructive.
It would be desirable to provide a rapid integrity test for ultrafiltration membranes which is both non-destructive and circumvents the tedious process of determining an entire effective pore size distribution for each membrane sample tested. Furthermore, it would be desirable to provide such a test which is independent of membrane surface area, porosity, and thickness; variables which can differ widely between membrane samples and often compound the difficulty in interpreting many integrity tests results. Additionally, it would be desirable to provide such a test which permits predicting with a high degree of accuracy whether a particular UF membrane is capable of retaining solutes of a given molecular size, such as biomolecules, while avoiding the need to actually challenge said membrane with a liquid solution containing the solute in order to make the determination.